When a rotating electrical machine winding is seated in a slot formed in a rotating electrical machine core in a rotating electrical machine, such as an induction motor or electric generator, that is rated at higher than 3 kV, the groundwall insulating layer is usually covered with a protective insulation made of a semiconductive material with a surface resistivity of about 1 kΩ so that the groundwall insulating layer is not damaged.
Insulation resin is grobal impregnated into the rotating electrical machine winding structured as described above in, for example, a vacuum state, while being disposed in the slot in the rotor core. When the impregnated insulation resin is then cured, the insulation of the rotating electrical machine winding is assured and no clearance is left between the winding and the core of the rotating electrical machine.
Since the rotating electrical machine winding and rotating electrical machine core have different coefficients of thermal expansion, thermal stress occurs between them during the curing of the impregnated insulation resin. Due to this thermal stress, peeling or a crack occurs in a vulnerable part on either of the groundwall insulating layer and protective insulation.
To address this problem, a semiconductive insulating layer with a separated layer between two components is proposed in, for example, Japanese International patent publication No. 11-509399 (Family: U.S. Pat. No. 5,945,764), which is formed near the rotating electrical machine core of a rotating electrical machine.